The Multi-Ethnic Study of Atherosclerosis (MESA) and the Jackson Heart Study (JHS) are both NHLBI-funded cohort studies that began recruitment in 2000 and were designed to investigate the pathogenesis of atherosclerosis in Americans of diverse races/ethnicities by providing accurate measurement of cardiovascular disease, its risk factors, and its outcomes. To enhance scientific productivity from these important studies, we propose to establish a joint MESA-JHS Atrial Fibrillation (AF) Working Group to foster research that brings together exam and events surveillance data with Medicare claims data. As MESA and JHS participants reach their late 60s and 70s, AF is becoming an important health problem. Events ascertainment in both studies already includes myocardial infarction, stroke, and heart failure, but AF ascertainment is underdeveloped. Based on our experience with identifying AF in the Cardiovascular Health Study, we will work with the MESA and JHS Coordinating Centers to prepare AF events files based on data from the cohort exams, events surveillance, and inpatient and outpatient Medicare claims data. The AF events files will be incorporated into the MESA and JHS databases, adding a critically important cardiovascular outcome. We propose specific aims related to AF that are focused and innovative, and for which existing and new data from MESA and JHS are poised to make novel contributions. These analyses will be undertaken in our AF Working Group by early stage investigators with mentorship from experienced investigators. First, we hypothesize that larger pericardial fat depots are associated with larger left atrial volume and with greater longitudinal left atrial enlargement. Second, we hypothesize that larger pericardial fat depots are associated with incident atrial fibrillation. Both analyses will be adjusted for obesity, hypertension, and other cardiovascular risk factors. If our hypotheses are correct, the results will motivate future studie of interventions or medications that affect lipid levels, lipid metabolism, weight, or blood glucos, and that may also alter pericardial fat deposition, potentially preventing progressive atrial remodeling and AF. The analyses of pericardial fat, longitudinal left atrial enlargement, and AF will serve as a springboard for additional analyses and ancillary studies related to AF from our Working Group. Our work incorporating Medicare claims data to identify AF events will also serve as a model for analyses of other health outcomes that can be readily identified using Medicare claims data. The Principal Investigator is an internationally recognized expert in the epidemiology of AF, with a track record of work using Medicare claims data in NHLBI cohorts, high productivity, and mentorship of graduate students and early stage investigators.